A power converter is a power processing circuit that converts an input voltage or current source waveform into a specified output voltage or current waveform. A full-bridge phase-shift pulse-width-modulated power converter (hereinafter referred to as a FPP converter) is a frequently employed switched-mode power converter that converts a direct current (DC) input waveform to a specified DC output waveform. The FPP converter generally includes switching circuitry coupled to an input source of electrical power. The switching circuitry includes two pairs of alternately conducting active switches. A primary winding of a transformer is coupled to the switching circuitry and a secondary winding of the transformer is coupled to a rectifier circuit (e.g., rectifying diodes). The rectifier circuit is coupled through an output filter to a load.
While the FPP converter employs the leakage inductance of the transformer to achieve zero-voltage switching (ZVS) across the active switches, other sources of inefficiencies exist in the FPP converter. More specifically, a parasitic capacitance in the form of the winding capacitance in the transformer and junction capacitance of the rectifying diodes resonate with the leakage inductance thereby inducing transients (e.g., ringing and voltage spikes) in the secondary side of the FPP converter. The transients are intensified in higher power and current applications. The transients are especially harmful to the rectifier circuit and noticeably affect the overall efficiency of the FPP converter.
There have been attempts in the past to minimize the effects of transients in power converters and the resulting stress on rectifier circuits. For instance, a resistor-capacitor-diode (RCD) snubber circuit is disclosed in "A 1 kW, 500 kHz Front-End Converter for a Distributed Power Supply System", by L.H. Mweene et al., Proc. IEEE Applied Power Electronics Conf., p. 423-432 (1989), which is incorporated herein by reference. The RCD snubber circuit not only effectively damps out oscillations in the rectifier's diode voltage, but also recovers a portion of the energy stored in the snubber capacitor to the output. During each switching transient, the reverse recovery energy due to the recovery process of the diodes is first stored in the snubber capacitor followed by a transfer of the energy to the output through the snubber resistor. During this process, some power is dissipated in the snubber resistor. As the output power increases, the power dissipated in the snubber resistor becomes significant thereby limiting the RCD snubber to low power applications. To reduce the power loss in the snubber resistor, a lossless snubber circuit was proposed in "High-Voltage, High-Power, ZVS, Full-Bridge PWM Converter Employing an Active Snubber" by J.A. Sabaste et al., 1991 VEPC Seminar Proc., pp. 158-162, which is incorporated herein by reference.
The lossless snubber circuit operates essentially the same as the RCD snubber circuit described above, except that the energy dumped into the snubber capacitor is recovered to the primary side inductor through an oscillation between the primary side inductor and the snubber capacitor, after the snubber switch is turned on. The snubber circuit will, however, lose its effectiveness when the converter is operated at very small duty ratios, which does not allow time for the energy stored in the snubber capacitor to discharge.
Another component readily employed to reduce the voltage stress on a rectifier circuit is a saturable reactor. A saturable reactor circuit is disclosed in "An Improved Zero-Voltage-Switched Pulse-Width-Modulated Converter Using a Saturable Inductor", by G. Hua et al., IEEE Power Electronics Specialists Conf. Rec., p. 189-194 (1991), which is incorporated herein by reference. Conventionally, a saturable reactor is series-coupled to each rectifying diode of the rectifier circuit. While the saturable reactor does a good job of reducing the reverse recovery of energy of the rectifier circuit, the saturable reactors exhibit losses that result in a relatively high temperature rise across the core and windings thereof. The temperature rise can be alleviated by employing several saturable reactors in parallel, but at the cost of valuable space on the printed circuit board and a prohibitively expensive saturable reactor circuit. As previously mentioned, the aforementioned circuits and other prior art circuits have inadequately dealt with the transients that adversely effect the rectifier circuit in power converters.
Accordingly, what is needed in the art is a snubber circuit for a rectifier circuit that minimizes a voltage stress thereacross to reduce the power losses associated with the rectifier circuit and oscillations in both voltage and current therefrom and is suitable for a vast range of power applications including higher power applications.